The present invention relates to the improvement of the load capability and rigidity of a linear guide assembly which receives loads through a number of balls which circulate, while rolling, in ball rolling passages extending through the guide rail and the slider slidable on the guide rail.
An example of this type of the linear guide assembly is disclosed in a Japanese Utility Model Examined Publication No. Hei. 6-646. In the disclosed linear guide assembly, at least three rows of ball rolling grooves, or upper, medium and lower ball rolling grooves, are formed in each side of a guide rail. A slider, which is slidable on the guide rail, has legs extending along both sides of the guide rail. Each of the legs of the slider includes ball rolling grooves respectively disposed in opposition to the ball rolling grooves of the guide rail, and ball circulating passages parallel to the ball rolling grooves thereof. A number of balls are put in the ball circulating passages. With movement of the slider, the balls circulate through the ball circulating passages while rolling therein. As shown in FIGS. 1(a) and 1(b), each of balls B contacts, at two points (T1 and T2), with the surface of a ball rolling groove of a guide rail 1 and the surface of a ball rolling groove of a slider 2, which is disposed so as to face the corresponding ball rolling groove of the guide rail 1. Prolonged lines L1 to L3 each connecting the contact points T1 and T2 are converged at intersection points O1 and O2, located inside the guide rail 1.
Another conventional linear guide assembly of the same type is disclosed in a Japanese Utility Model Unexamined Publication No. Sho. 64-53622. As shown in FIGS. 2(a) and 2(b), the linear guide assembly has also a three-row ball rolling groove structure. In the passages defined by the upper and lower ball rolling grooves of the slider and the guide rail, which are confronted with each other, each ball contacts, at two points T1 and T2, with the surfaces of its associated ball rolling grooves being confronted with each other. In the passage defined by the medium ball rolling grooves facing each other, each ball contacts, at four points T1 to T4, with the surfaces of the ball rolling grooves of the guide rail and the slider.
When the intersection points (converging points) of the prolonged lines connecting the contact points of the balls with the groove surfaces are located inside the guide rail, an elastic displacement of the linear guide assembly is increased against a moment load acting on the slider so as to roll the slider, and the linear guide assembly exhibits an self-aligning function. On the other hand, when the converging points are located outside the guide rail, the linear guide assembly exhibits a high rigidity against the moment load.
In the Japanese Utility Model Examiner Publication No. Hei. 6-646 shown in FIGS. 1(a) and 1(b), its load capacity is limited to a load capacity corresponding to only two rows of ball rolling grooves. The reason for this is that when a load F acts on the upper side of the slider 2 (FIG. 1(a)) or a load f acts on the lower side thereof (FIG. 1(b)), the load is received by one or two rows of ball rolling grooves.
The load capacity of the Japanese Utility Model Unexamined Publication No. Sho. 64-53622, as shown in FIG. 2(a), is limited to that corresponding to two rows of the ball rolling grooves as of the Japanese Utility Model Examiner Publication No. Hei. 6-646 shown in FIGS. 1(a) and 1(b). The linear guide assembly may be constructed such that the load is received by the upper, medium and lower ball rolling grooves, as shown in FIG. 2(b). In this case, the intersection points O1 and O2 of the prolonged lines L1 to L3 connecting the contact points T1 to T4 are located separately inside and outside the guide rail 1, and the self-aligning function fails to operate and the rigidity against the moment load is low. In this respect, this is impractical.
Further, in an actual use of the linear guide assembly, the upward and downward loads acting on the linear guide assembly are generally different in their magnitudes. To increase the rigidity of the linear guide assembly against an excessively load acting thereon, a measure may be taken in which an increased pressure is merely applied in advance to the linear guide assembly may be made. However, the measure results in an excessive increase of the prepressure, possibly leading to the damage of the linear guide assembly.
In the Japanese Utility Model Unexamined Publication No. Sho. 64-53622, there is no description on a ratio of the radius of curvature of the flank of the groove of the ball rolling passage to the diameter of the ball (the ratio will be referred to as a groove R ratio), although the linear guide assembly of the publication is unique in that the ball contacts at four points with the groove surfaces in the medium ball rolling passage, and the ball contacts at two points with the groove surfaces in the upper and lower ball rolling passages. Incidently, in the conventional linear guide assembly, referred to above, which has two rows of ball rolling passages, the groove R ratios of the two rows of ball rolling passages are equal. The conventional linear guide assembly having three rows of ball rolling passages has not yet been put into practice. In designing this linear guide assembly for its actual use, the structure of the linear guide assembly having two rows of ball rolling passages will be directly applied to the linear guide assembly.
The load capacity and rigidity of the linear guide assembly become large with increase of the contact area of the ball and the ball rolling passage. To increase the load capacity and rigidity at a fixed ball size, it is only needed to reduce the radius of curvature of the flank of the ball rolling passage.
FIG. 3 is a diagram showing the relationship between the groove R ratios and contact angles of the flanks of the ball rolling grooves, which define the ball rolling passages in which each ball contacts with the groove surfaces at two points. In the ball rolling passage shown in FIG. 3(a), the flank f of the ball rolling groove Ma has the radius of curvature R1. In the ball rolling passage shown in FIG. 3(b), the flank f of the ball rolling groove Mb has the radius of curvature R2. Here, R1&gt;R2. Since the radius of curvature is small (a groove R ratio of the radius of curvature of the flank f to the diameter of the ball 210 is small), a contact area Sb, elliptical in shape, of the ball rolling groove Mb where it contacts with the ball 210 may be larger than a contact area Sa, elliptical in shape, of the ball rolling groove Ma (Sb&gt;Sa). If the contact area is large, the load capacity and rigidity of the linear guide assembly are increased. The reason why the contact area is elliptical is that the ball rolling grooves Ma and Mb are linear in the direction vertical to the paper of the drawing.
The relationship between the groove R ratios (the radii of curvature) and contact angles of the flanks of the ball rolling grooves, which define the ball rolling passage in which each ball contacts with the groove surfaces at four points, will be described with reference to FIG. 4. In FIG. 4, for ease of explanation, the curvature radii R1 and R2 of the right and left flanks fL and fR of a Gothic arch groove Mg are different from each other; R1 (left)&gt;R2 (right). Let an initial contact angle .theta. be 45.degree. for both the flanks fL and fR (flanks indicated by one-dot chain lines). In this case, the center of the curvature of the left flank fL is O1, and that of the right flank fR is O2.
The curvature centers O1 and O2 of the flanks fL and fR are displaced to positions O1' and O2', respectively. For a change quantity a of the contact angle .theta. of the ball 210, a change quantity .alpha.1 of the contact angle of the ball on the Iright flank having the curvature radius R2 is much greater than a change quantity .alpha.2 of the contact angle on the left flank having the curvature radius R1, although the displacement quantities (offset values) A1 and A2 of the curvature centers are equal to each other.
Thus, in the ball rolling passage having four contact points, contact conditions of the ball and the ball rolling grooves are easy to change, so that basic characteristics of the linear guide assembly, such as load capacity, rigidity and rolling frictional force, also change.
Where the radius of curvature of the ball rolling groove of the ball rolling passage having four contact points is small, a small error of the contact angle of the ball on the ball rolling groove greatly affects the function and characteristics of the linear guide assembly. For this reason, the accuracy control in working the product is difficult. Where four contact points are used and the radius of curvature is small, the contact area is large and the slide is great. Where the radius of curvature of the ball rolling groove of the ball rolling passage having four contact points is large, the load capacity and the rigidity of the resultant linear guide assembly are in unsatisfactory levels. Thus, a designer encounters an antinomic problem in designing the linear guide assembly.